100 exercises to learn Rust

book/src/03_ticket_v1/04_visibility.md

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+# Visibility
+
+When you start breaking down your code into multiple modules, you need to start thinking about **visibility**. 
+Visibility determines which regions of your code (or other people's code) can access a given entity,
+be it a struct, a function, a field, etc.
+
+## Private by default
+
+By default, everything in Rust is **private**.  
+A private entity can only be accessed:
+
+1. within the same module where it's defined, or
+2. by one of its submodules
+
+We've used this extensively in the previous exercises:
+
+- `create_todo_ticket` worked (once you added a `use` statement) because `helpers` is a submodule of the crate root,
+  where `Ticket` is defined. Therefore, `create_todo_ticket` can access `Ticket` without any issues even
+  though `Ticket` is private. 
+- All our unit tests are defined in a submodule of the code they're testing, so they can access everything without 
+  restrictions. 
+
+## Visibility modifiers
+
+You can modify the default visibility of an entity using a **visibility modifier**.  
+Some common visibility modifiers are:
+
+- `pub`: makes the entity **public**, i.e. accessible from outside the module where it's defined, potentially from 
+  other crates.
+- `pub(crate)`: makes the entity public within the same **crate**, but not outside of it.
+- `pub(super)`: makes the entity public within the parent module.
+- `pub(in path::to::module)`: makes the entity public within the specified module.
+
+You can use these modifiers on modules, structs, functions, fields, etc.
+For example:
+
+```rust
+pub struct Configuration {
+    pub(crate) version: u32,
+    active: bool,
+}
+```
+
+`Configuration` is public, but you can only access the `version` field from within the same crate.
+The `active` field, instead, is private and can only be accessed from within the same module or one of its submodules.
+
+## References
+
+- The exercise for this section is located in `exercises/03_ticket_v1/04_visibility`